1. Field of the Invention
The invention relates to a control apparatus for a high-pressure fuel pump for an internal combustion engine which periodically repeats an intake stroke in which fuel is sucked from a low-pressure side into a pump chamber by increasing a volume in the pump chamber, and a compression stroke in which fuel in the pump chamber is compressed by reducing a volume in the pump chamber, and which adjusts an amount of fuel that is force-fed to the side of a fuel injection valve by opening or closing a spill valve during the compression stroke.
2. Description of Related Art
In an internal combustion engine (e.g., a direct-fuel-injection-type internal combustion engine) in which fuel injection valves are employed to supply combustion chambers with a required amount of fuel during a compression stroke, a precise amount of fuel needs to be injected from the fuel injection valves to cause desired combustion. In order to inject a precise amount of fuel, a fuel pressure for the fuel injection valves (e.g., a fuel pressure in a fuel distribution pipe for distributing fuel to the fuel injection valves) needs to be adjusted to a suitable pressure.
For this adjustment of a fuel pressure, a sufficient amount of fuel needs to be supplied separately to the fuel distribution pipe from a fuel pump in such a manner as to supplement an amount of fuel injected from the fuel injection valves (an amount of consumption). To realize this, it is known to perform an operation in which fuel of an amount exceeding a maximum amount of fuel injected from the fuel injection valves is constantly supplied to the fuel distribution pipe so that the fuel pressure does not decrease due to a deficiency of fuel no matter how much fuel is actually injected from the fuel injection valves (even though a smaller amount of fuel than the maximum fuel injection amount is injected from the fuel injection valves). In this case, a surplus of fuel that has not been injected from the fuel injection valves is again returned to a fuel pump or to a fuel tank through a relief valve provided in the fuel distribution pipe.
However, if an excessive amount of fuel is constantly supplied to the fuel distribution pipe and caused to circulate therethrough, a large amount of energy is required in accordance with a surplus of fuel that has been supplied. This leads to deterioration of fuel consumption of the internal combustion engine.
Thus, with a view to preventing such waste of energy, there has been proposed an operation in which fuel of an amount that just supplements an amount of fuel injected from fuel injection valves is supplied from a fuel pump to a fuel distribution pipe (see, e.g., Japanese Patent Application Laid-Open No. 4-50462). In this application, a feed-forward term based on a fuel injection amount is first calculated to set a period for openingly driving an electromagnetic spill valve that is provided in a high-pressure fuel pump for force-feeding fuel into the fuel distribution pipe. Then, a feedback term based on a difference between a fuel pressure and a target fuel pressure is added to the feed-forward term, and a required control amount is set as a ratio (duty) of the period for openingly driving the electromagnetic valve. This control makes the amount of fuel supplied to the fuel distribution pipe neither excessive nor deficient. This eliminates the necessity to constantly cause a large amount of fuel to circulate through the fuel distribution pipe. As a result, the energy required for fuel supply control decreases, and it becomes possible to prevent deterioration of fuel consumption of the internal combustion engine.
However, according to the aforementioned control operation, if the fuel injection amount becomes small (e.g., at the time of idling), the feedback term gradually decreases in response to a decrease in the feed-forward term. Eventually there may arise a situation where the calculated duty ratio frequently becomes equal to zero (0%). If the duty ratio becomes equal to zero, the electromagnetic spill valve is stopped from being closingly driven. Therefore, operation noise resulting from a closing motion of the electromagnetic spill valve frequently ceases. Because such cessation of the closing motion of the spill valve results from the feedback term, it is not possible to ensure that the spill valve is stopped periodically. As a result, irregular cessation of a closing motion of the spill valve brings about generation of irregular operation noise audible to the driver. Thus, the driver may feel a sense of incongruity.